Spiny Spiders Use Colors to Lure Lunch

June 20

By Lee Dye

While studying animal behavior in the rain forest of northern Australia, Mark Hauber couldn't help but notice spiders with brightly colored stripes on their bodies that stood out like neon warning lights. How, he wondered, could they catch lunch?

It was intriguing to the Cornell University researcher because normally spiders need to blend into the background as they lay in wait for some unsuspecting insect.

"They were really conspicuous," Hauber says.

These spiders went to all the trouble of building an inconspicuous web, and then they stand out in the middle of the web as a "really bright spot," he said. "It just doesn't make any sense."

Hauber, a researcher in Cornell's Department of Neurobiology and Behavior, set up a simple experiment to figure out what the spiny spiders (Gasteracantha fornicata) were up to. Although other research and even common sense indicated that members of the animal kingdom use color to attract a mate, or blend in with the background, these critters were using color for an altogether different purpose.

Color, says Hauber, was part of the trap. They use color to lure in their lunch, and that's believed to be very rare in the animal world.

Looking Pretty for Lunch

It's not clear yet just why it works, but Hauber suspects that to the unwary insect, the spider looks like a flower, not a bloodsucking carnivore.

But bright colors would also alert other predators, like birds and spider-eating mammals, to the presence of the spider. Having a bright stripe running down your back is not exactly a good thing if you're trying to hide from something else.

But it apparently works for the spiny spider, Hauber argues in a report in an upcoming issue of the Royal Entomological Society's journal, Ecological Entomology, because birds and other predators have learned that it's not worth the effort to try and eat one of the little beasts.

The spiders have a hard crust with spines on both sides, "so it's hard to chew and not really easy to eat by a predator," he says. "So it can be conspicuous, and once it's conspicuous, it can use that color to attract the insects."

Hauber, who was interviewed while driving to his new post at the University of California, Berkeley, carried out a simple but cleaver bit of research to reach his conclusions. He used a felt-tipped pen to blacken the color on some spiders, and left others in their natural cloaks of yellow and black stripes.

Then, he kept tabs on just how successful the two groups of spiders were at capturing insects.

"The prey-catching rate was calculated on an hourly basis," he says.

It turns out that the spiders that had been robbed of their color had a heck of a time catching insects.

"Their catching rate decreased to about zero," Hauber says.

But the other spiders, the ones with the bright colors, caught all the insects they could eat.

So, presto, color is the key, he says.

Spider of Many Stripes

Hauber admits his findings are not all that popular with other animal behaviorists. Some argue that anything other than inconspicuousness would work against the spider's interest, because in time the insects would learn to avoid flowers that could actually be spiders.

But Hauber argues that it would be too "costly" for the insects to do that because they depend on similar flowers for survival, and most of the time what looks like a flower would indeed be a flower, not a spider.

And besides that, the spiny spider is a very tricky chap indeed. Although they are found all over the world, they differ in colors and patterns, so the yellow-striped spiders Hauber studied in Australia are very different in appearance from other spiny spiders.

"They come in such variations that the insects can't learn what the spiders look like," he says. "From the same mother you could have different looking progeny coming out, so there isn't a clear phenotype where you could say this is what a spiny spider looks like, so avoid it."

That gives the spiny spider a leg up on most other spiders. Most spiders build their webs near lights, and use the lights to attract prey. It's sort of a passive-aggressive system.

But the spiny spider of northern Australia, and possibly elsewhere, doesn't have to do that.

"It's got it's own attracting device," Hauber says.

It's a jungle out there.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.